1. Field of the Invention
This invention pertains generally to fabricating nanotubes, and more particularly to a method of fabricating a nanotube over a sacrificial nanowire template.
2. Description of Related Art
Since the discovery of carbon nanotubes (see lijima, S., Helical microtubules of graphitic carbon, Nature, 354, 56 (1991), incorporated herein by reference), there have been significant research efforts devoted to nanoscale tubular forms of various solids (see Tenne, R. & Zettl, A. K., Nanotubes from inorganic materials, Top. Appl. Phys. 80, 81–112 (2001); Tenne, R., Inorganic nanoclusters with fluorine-like structure and nanotubes, Prig. Inure. Chem. 50,269–315 (2001); Partake, G. R., Cromlech, F. & Nester, R., Oxidic nanotubes and nanorods—Anisotropic modules for a future nanotechnology, Angew. Chem. Int. Ed. 41, 2446–2461 (2002); Martin, C. R., Nanomaterials—a membrane-based synthetic approach, Science, 266, 1961–65 (1994); Ajayan, P. M. et al., Carbon nanotubes as removable templates for metal-oxide nanocomposites and nanostructures, Nature, 375, 564–566 (1996); Yang S. M. et al., Formation of hollow helicoids in mesoporous silica: Supramolecular Origami, Adv. Mater. 11,1427–30 (1999); Kondo, Y. & Takanayagi, K., Synthesis and characterization of helical multi-shell gold nanowires, Science, 289, 606–608 (2000); Li Y. et al., Bismuth nanotubes, J. Am. Chem. Soc. 123, 9904–05 (2001); and Wu, Y. & Yang, P., Melting and welding semiconductor nanowires in nanotubes, Adv. Mater. 13, 520–523 (2001), the above references being incorporated herein by reference).
The formation of tubular nanostructures generally requires a layered or anisotropic crystal structure (see Tenne, R. & Zettl, A. K., Nanotubes from inorganic materials, Top. Appl. Phys. 80, 81–112 (2001); Tenne, R., Inorganic nanoclusters with fluorine-like structure and nanotubes, Prig. Inure. Chem. 50, 269–315 (2001); Partake, G. R., Cromlech, F. & Nester, R., Oxidic nanotubes and nanorods—Anisotropic modules for a future nanotechnology, Angew. Chem. Int. Ed. 41, 2446–2461 (2002), the preceding references incorporated herein by reference).
There are reports of nanotube formation of solids lacking layered crystal structures, such as silica, alumina, silicon and metals through templating of carbon nanotubes and porous membranes or thin film rolling Schmidt, O. G. & Eberl, K., Thin solid films roll up into nanotubes, Nature, 410, 168 (2001), incorporated herein by reference).
The nanotubes produced by the above methods, however, are either amorphous, polycrystalline, or they exist only in ultra-high vacuum environments.
The significance of hollow inorganic nanotubes is being recognized and they have wide applicability in bioanalysis and catalysis (see Lee, S. B.; Mitcell, D. T.; Trofin, L.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. Science 2002, 296, 2198, incorporated herein by reference). Among these hollow nanotubes silica nanotubes are of special interest because of their hydrophilic nature, colloidal suspension formation, and surface functionalization accessibility for both inner and outer walls. These modified silica nanotubes and nanotube membranes for example have applicability for bioseparation and biocatalysis (see Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N. C.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864, incorporated herein by reference).
Recently, bright visible photoluminescence from sol-gel template synthesized silica nanotubes was observed by Zhang et al. (see Zhang, M.; Ciocan, E.; Bando, Y.; Wada, K.; Cheng, L. L.; Pirouz, P. Appl. Phys. Lett. 2002, 80, 491; incorporated herein by reference). In addition, the study of the physical and chemical nature of molecules or ions confined within the inorganic nanotubes is of great current interest.
Silica nanotubes have been synthesized typically within the pores of porous alumina membrane templates using the sol-gel coating technique (see Martin, C. R. Chem. Mater. 1996, 8, 1739, incorporated herein by reference). Alumina templates can be dissolved to liberate single silica nanotubes. These nanotubes prepared at low temperature have porous walls and are relatively fragile. Once the templates are removed, the silica nanotubes will generally bundle up and become less oriented. The same applies to the silica nanotubes prepared at low temperature using other templates (see Obare, S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2001, 1, 601; Jung, J. H.; Shinkai, S.; Shimizu, T. Nano Lett. 2002, 2, 17; Yin, Y. D.; Lu, Y.; Sun, Y. G.; Xia, Y. N. Nano Left. 2002, 2, 427, incorporated herein by reference).
Accordingly, the growth of single-crystalline semiconductor nanotubes provides a number of advantages for nanoscale electronics, optoelectronics, and biochemical sensing applications. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed nanoscale growth methods.